Acquired Immunodeficiency Syndrome, commonly known as AIDS, is a disease that currently plagues millions of people worldwide. Scientists isolated human immunodeficiency virus (HIV), the virus that causes AIDS, in 1983, and have tried to develop cures and therapies for this devastating virus. Unfortunately, there is no known cure. Research has produced a number of drugs that treat the disease in the hopes of prolonging the length and quality of life for HIV-infected individuals. HIV infection, however, is particularly difficult to treat because the virus rapidly mutates into different forms, and each form may respond differently to drugs and therapies. As a result, scientists have developed a number of different drugs that attack HIV in different ways, although new and effective treatments are desperately needed.
To understand how current HIV treatments work, it is helpful to know how HIV reproduces. HIV is an RNA virus, that is, its genes are coded on strands of RNA. The first step in the life cycle of HIV is the virus's entry into a host cell. In the second step, HIV makes a DNA copy of its genes from the RNA template using a viral enzyme called reverse transcriptase. Third, the DNA copy of the viral gene is inserted into the host cell's own DNA genes. This means that the infected cell now has both its own DNA and HIV DNA in its genome. Cells prepare RNA copies of the DNA in their genes by a process known as "transcription," and so the fourth step involves transcribing RNA--not only from the host cell's own DNA, but from the viral DNA that has become part of the host cell's genome. Some of this newly transcribed RNA is the genetic material that goes into new HIV viruses, while other viral RNA is used in the fifth step to make proteins and enzymes that enable the creation of new HIV viruses. Sixth, new viruses are assembled in the cell using the viral RNA and the proteins and enzymes that the cell has produced. Lastly, newly formed HIV viruses leave the host cell to infect other cells and continue multiplying.
Twelve drugs are currently approved in the United States for treating HIV; each one fights the virus in a different fashion. These drugs fall into two general categories-reverse transcriptase inhibitors and protease inhibitors.
Reverse transcriptase inhibitors attack the second step of the virus's life cycle, that is, when the virus makes a DNA copy of its RNA genome using reverse transcriptase. Reverse transcriptase takes the building blocks of DNA, called nucleotides, and bonds them together (through a phosphodiester linkage) in a specific sequence using the viral RNA as a template. The resulting DNA is called a "provirus," which is inserted into the host cell's genome. However, as the reverse transcriptase makes DNA from the RNA, it often makes mistakes. It is these "mistakes" that create so many different forms of HIV, making it harder to develop effective treatments. However, it is precisely this tendency to make mistakes that enables scientists to treat HIV with inhibitors of reverse transcriptase. For example, many reverse transcriptase inhibitors operate through defective nucleotides that reverse transcriptase uses in building viral DNA. When reverse transcriptase inserts these defective nucleotides into the growing DNA chain, the defective nucleotides are unable to bond with other nucleotides and so the enzyme stops building the chain. The result is that reverse transcriptase cannot make a complete viral DNA molecule.
These defective nucleotides are called dideoxynucleotides, literally, nucleotides without two oxygens. One of the most successful dideoxynucleosides is azidothymidine (AZT). AZT has a benefit in that HIV reverse transcriptase incorporates AZT into the growing viral DNA, while the host cell's own DNA-generating machinery does not incorporate this defective nucleotide. This enables the replication of host cell DNA to continue relatively unaffected by the presence of AZT.
Other forms of reverse transcriptase inhibitor do not operate by providing defective nucleotides to the RT. Instead, such inhibitors bind to a certain part of the enzyme when it is complexed to DNA. It is believed that this slows down the rate at which the viral DNA is made. Unfortunately, resistant reverse transcriptase has been identified.
The main drawback of reverse transcriptase inhibitors in general, and AZT in particular, is that they can be extremely toxic to the person under treatment. Therefore their dosage must be limited and monitored. In addition, HIV can mutate to create viruses that are resistant to these treatments. Consequently, patients now take combinations of dideoxynucleotides to reduce the chance of developing drug resistant forms of HIV. New treatments that could reduce the patient's reliance on these toxic treatments are desperately needed.
In contrast to nucleoside and non-nucleoside inhibitors of reverse transcriptase, HIV protease inhibitors target the fifth step of the virus's life cycle--when the virus causes the host cell to make proteins that are used in assembling new viruses. HIV protease is a viral enzyme that cuts large proteins (called "polyproteins") produced from viral genes into smaller proteins, such as viral coat proteins and viral enzymes, including reverse transcriptase and the viral protease. HIV protease selectively binds to a site on the polyprotein to be cut (the "substrate"), and then cuts the polyprotein in to smaller proteins. HIV protease inhibitors are effective because the protease will bind with the inhibitor, and attempt to cut it. However, the inhibitor cannot be cut and stays bound to the protease. In doing so, the true substrates cannot gain access to the enzyme. If the polyproteins are not cut, then the smaller proteins necessary for the synthesis and assembly of viral particles are not formed. The resultant defective virions prevent further HIV infection.
Despite recent advances in combination chemotherapy using both reverse transcriptase and protease inhibitors, no cures are claimed and resistance is beginning to develop. New drugs having unique structures and targets are desperately needed.
People with AIDS typically suffer infection by opportunistic organisms. One such organism is human cytomegalovirus ("HCMV"). HCMV is a DNA virus; that is, HCMV's genome is a DNA molecule. HCMV is most commonly seen in AIDS victims and is believed to take advantage of the victim's weakened immune system. HCMV infections often lead to death or severe disease, such as blindness. At present, very few drugs have proven effective, but current treatments include ganciclovir, foscarnet, and cidofovir. Similar to HIV, HCMV makes DNA copies of its genome once it has infected the host cell. Unlike HIV, HCMV uses its genetic DNA as a template, and uses an enzyme called a polymerase to make the new DNA chains. Both ganciclovir and cidofovir operate by binding to this polymerase and causing a slowing and eventually stopping the DNA chain elongation when incorporated into the viral DNA.
By contrast, foscarnet does not incorporate into the growing viral DNA chain, but instead blocks a binding site of the polymerase, inhibiting the growth of the DNA chain. Unfortunately, none of these treatments provides a cure for HCMV, and all have significant drawbacks. These treatments can be very toxic, and drug resistant strains of HCMV develop within a relatively short period of time through mutations in one or more of the virus's genes. As will be readily appreciated, novel treatments against DNA viruses generally, and human cytomegalovirus in particular, are desperately needed.
Relatively recently, polyribonucleotides and oligoribonucleotides containing sulfur have been shown to be potent against both HIV and HCMV. Regarding HIV, the working hypothesis is that these drugs bind to and fill the RNA-binding site on reverse transcriptase. The method of DNA antiviral activity is not yet known, but it is thought that the amphipathic character (hydrophobic, or water-fearing, base and hydrophilic, or water-loving, backbone), and the ability to form a highly ordered structure in solution are prerequisites to antiviral activity. The drawback of these compounds is that they are not readily made via chemical or enzymatic techniques, and it is believed that they rely on secondary structure in solution for antiviral activity.
Consequently, it would be a great advancement in the art to provide treatments with potent broad spectrum antiviral activity against both DNA viruses and RNA viruses. It would be a further advancement in the art to provide a composition showing antiviral activity against HIV. Still another advancement in the art would be to provide a composition showing antiviral activity against HCMV. Finally, it would be a great advancement in the art to provide compositions that show potent activity against both HIV and HCMV, thereby reducing the reliance on toxic treatments now in use, and decreasing the chance of creating resistant strains of HIV and HCMV.
Such compositions and their methods of manufacture and use are disclosed herein.